Summary Northwest Airlines Inc. Flight909, a Boeing 747-200 freighter (registration N632NW, serial number 23112) was en route from Wilmington, Ohio, to Anchorage, Alaska, United States, with a crew of three on board when the crew noticed smoke and fire emanating from a cockpit circuit breaker panel located behind the second officer's position. The crew completed the related emergency cockpit checklist procedures and declared an emergency with air traffic control. The fire self-extinguished during the action of the checklist procedures. The aircraft diverted to Winnipeg, Manitoba, landing safely at 0649 central daylight time with airport rescue and fire-fighting vehicles standing by. There were no injuries. Ce rapport est galement disponible en franais. Other Factual Information A special aerodrome weather report was issued for Winnipeg at 0622 central daylight time.1 The weather was as follows: wind 320true(T) at 12knots, visibility 15statute miles with a few clouds at 1100feet, broken clouds at 2200and 9500feet, and a temperature of 12C. The weather, both at cruise altitude and at landing, did not play a factor in this occurrence. At the time of the occurrence, the aircraft was in cruise flight at flight level360. At approximately 0625, the second officer noticed a fluctuation in the number four engine fire-detection loop A nacelle temperature indicator. The fluctuation increased in intensity, followed by a blinking fault light on the engine fire-detection panel. The fault light eventually illuminated steady, along with a master fire-detection light in the cockpit annunciator panel. The crew accomplished the Engine Fire Detection Light and/or Fault Light Illuminated company operations manual (COM) Section2.26.32 checklist in the emergency/abnormal procedures manual. The fire-detection loop was selected from the loop A to the loop B position and the fault light extinguished. Soon after the completion of the checklist, an electrical smell was detected. The second officer confirmed that the smell was coming from behind the P6panel located at the rear of the cockpit. The second officer looked through the designed viewing ports and saw flames and smoke within the panel. The crew members donned their oxygen masks and smoke goggles and control of the aircraft was given to the first officer. The first officer declared an emergency and requested a direct clearance to Winnipeg. The captain and second officer began the Electrical Fire or Smoke COM Section2.26.6 checklist, during which time the Engine Fire Detect 3A-4A circuit breaker on the P6panel popped. The fire began to increase in intensity and stopped only after the crew tripped the number one and number two generator breakers in accordance with the checklist instructions. The fire damage was contained within the interior of the P6panel and the visibility inside the cockpit was normal at all times. When the fire was out, the crew members removed their oxygen masks and smoke goggles. The captain took control of the aircraft and approximately 10000pounds of fuel was dumped to bring the aircraft down to its maximum landing weight. The fuel dump was terminated above 10000feet above sea level and a visual approach was made to Runway36 at the James Armstrong Richardson International Airport in Winnipeg, where a safe landing was accomplished. The aircraft was inspected and the fire was found to originate from within the number four engine loop A (A4) circuit board warning card mounted in the fire-detection control electronics box located on the aft side of the P6circuit breaker panel (see Photo1). The fire had spread to both the adjoining A3and A5circuit cards, as well as to a major aircraft wiring bundle located above the control box. Maintenance personnel replaced the control box and the circuit cards and repaired the damaged wiring bundle. The number four engine was started and the number four engine fire-detection loop A nacelle temperature indicator again began to fluctuate as in the original occurrence. The fault light on the engine fire-detection panel illuminated and the engine was shut down. The A4circuit card was again found to be damaged, indicating that both card faults were likely caused by stray voltage entering the wiring harness from outside the control box, causing the cards to overheat. There is no circuit breaker protection in the circuit between the circuit board warning card and the engine fire loop. The number four engine fire-detection system electrical firewall disconnect plug was removed. The inner rubber insulator, used to isolate the individual wiring pins, was found to be deteriorated and saturated with Skydrol hydraulic fluid. The electrical plug was replaced and all systems tests and engine ground runs were considered normal. The aircraft was prepared for flight and, shortly after take-off, the number four engine fire-detection loop A nacelle temperature indicator pegged to the top of the indicator scale, and the fault light and master fire-detection light again illuminated. The aircraft returned to Winnipeg and the A4circuit card was again found to be damaged. The aircraft was prepared for a three-engine ferry (with the number four engine inoperative) back to the operator's main base in Minneapolis, Minnesota, United States, where greater ground support could be offered. During troubleshooting in Minneapolis, alternating current voltage (VAC) was noted to be present on the number four engine loop A direct current (VDC) circuit. The voltage stopped when the generator constant speed drive (CSD) was disconnected. The wiring near the generator was inspected and four wires in a wire bundle were found to be chafed; three of the wires were chafed to the conductor core. Two of the wires chafed to the conductor core originated at pins 1and 3at connector DG1B on the permanent magnetic generator (PMG). With the CSD coupled and the engine running, the PMG produces approximately 60VAC. The remaining wire was a 28VDC voltage wire (W266-W13-16R), which is part of the engine fire-detection loop. The chafed wires were bound together with plastic tie wraps. When the tie wrap was removed, it was noted that a slack portion of the engine fire-detection wire loop had been folded back and secured to the adjacent PMG wiring harness (see Photo2). During engine assembly, the harnesses are installed as part of the Northwest Airlines Inc. quick exchange component (QEC) shop form QEC-42. The harnesses are assembled and installed using guidelines outlined in Northwest Airlines Publication 25092, the Boeing Standard Wiring Practices (Chapter20), and the Boeing Powerplant Buildup manual of standard practices (Section 71-00-00) for a level3 high-vibration area (see AppendixA). These documents also provide guidance regarding harness routing and best assembly and installation practices. Chapter 20-10-11, paragraph3C, specifies, in part, that if it is possible, all wires must be parallel before a wire harness tie is assembled on the wire harness. Photo 3. Example of good wire lacing and routing Northwest Airlines Inc. consulted technicians who had previously worked on the engine. The technicians indicated that the wire routing configuration and bundling was at variance with QEC shop procedures (see Photo3). When the engine is overhauled, the harness is removed for overhaul inspection as per the component maintenance manual. The general instruction is to disassemble as required to accomplish cleaning, inspection, and repair. All other shop visits would only require a visual inspection/check as installed. Maintenance records indicated that the number four JT9D-7R4G2 engine, serial number715053, was last overhauled by Northwest Airlines Inc. in October2002 and had accumulated 13376hours and 2339cycles since install. There was no record of engine repairs since the installation. An under-cowl maintenance check was accomplished 383hours before the occurrence, coincident with a 1Acheck. The time remaining until the next inspection was 216hours. The under-cowl check calls in part for the inspection of electrical cables for chafing, loose connections, loose back-shells, burned spots and deterioration. The occurrence aircraft was used by Northwest Airlines Inc. on cargo flights worldwide and had maintenance work completed in many cities and countries around the world. The damaged fire-detection control electronics box along with the digital flight data recorder (DFDR) and the cockpit voice recorder (CVR) were forwarded to the TSB Engineering Laboratory in Ottawa, Ontario, for examination. The DFDR and CVR did not provide any valuable information. The DFDR only recorded a limited number of parameters, none of which related directly to the fire. The two-hour CVR recording had been overwritten by ground operations after the flight. The number four engine loop A (A4) circuit board warning card (part number 60B00023-96) in the fire-detection control electronics box was assessed as being the source of the fire. The power resistor (R11) on the side of the board had burnt and burst from internal overheating due to excessive current passing through the resistor.